Resistance strip and method of making same



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Marh 28, 1933.

N. c. scawiELLENGL-al.\.93p142 RESISTANCE STRIP METHOD OF' MAKING ,SAME' Filed April 30, 1950 75 iff/f /2 Patented Mar,v 28, 1933- l-n'fnrrso STATES PATENT@ OFFICEl NEWTON C. SCHELLENGER, 0F ELKHART, INDIANA, ASSIGNOR T0 CHICAGO TELEPHONE SUPPLY COMPANY, OF ELKHART,

INDIANA, .A CORPORATION OF INDIANA RESISTANCE STRIP' AND METHOD OF IdAKING SAME Application led April 30,

This invention relates o resistance strips and to the method of making such strips.

Resistance units m the iorm of rheostats 'containing a high resistance Wire wound element and suitable -contacting means forengaging that element to vary the resistance of a circuit, are commonly used as volume controls for radio sets, control resistors for X-ray apparatus, and other similar uses. l'norder that the control devices may be made compact-l ly, it is customary to wind the resistance element with a wire of very small diameter and composed of a nickel or nickel alloy which has a high specific resistance.

I The wire of the elementis usually wound upon a strip of insulating material, which strip is usually bent into an arcuate form and mounted in a suitable frame to make up the completed rheostat. v

Because ot the smallness of the wire used in elements of this kind, it is difficult to securely fasten the ends of the Wireto the resistance strip so that it will not unravel or unwind therefrom during the handling of the strip in the manufacture of the rheostat. Various arrangements have'heen tried heretofore to securely fasten the ends of the Wire,

but insofar as l am aware each of these arrangements has a serious drawback in that the mechanical lor electrical properties of the resistance strip are impaired thereby, and in certain instances the cost of manufacture is made unduly high hy their use. Because of the nickel content of the Wire-used in elements of this kind, it is not practical to Jfasten the wire hy soldering the end "turns of the units together,

y In my present invention, l provide a new vand improved method of securely holding the end turns of such a resistance unit together so that they cannot unwind from the insul ating st upon which they are placed. ln the pret *tuent of my invention, thelast lier .v oedded in asoft metal such as lean or o similar metal, so that the metal practica L, completely encases these turns. v

This arrangement provides a simple yet highly satisfactory method or' fastening the 1930. Serial No. 448,520.

ends of a small resistance Wire in place on a resistance strip.

In certain resistors of this-type, particularly those used as volume controls in radio sets and v other similar devices, it is often advantageous to have the cement designed to givea low resistance progression when the contactor is placed in the immediate vicinity of the ends of the element; that is, when the contacter which engages the resistance element is resting in itseXt-reme end position, the resistance ot' the element engaged thereby is preferably very low. As the contacter is turned away from this point, preferably the addition of resistance is very gradual at first, so that the volume control will have what is knownv in thev trade as a smooth takeoff, and can therefore be turned from its zero position without producing undesirable extraneous noises in Y the radio set.

My present invention also provides a sim- 4ple'and efficient method of tapering the end turns of a resistance elementof this type, so

to maximum at a point several turns away from the end turn of the element.

lt will be noted that, by this varying crosssection of the metal. foil, the length of each individual turn of resistance wire is progressively diminished as the end of the strip is approached. rl`his provides a low resistanc gradient adjacent the ends of the resistance strip, which is desirable in some cases. rlhe gradually increasing length of Wire from the edge of the strip upon which the contacter arm engages to the foil progressively increases the resistance of the turns of wire as the Contact moves away from the terminal,

and thus provides a graduated increase of resistance per unit of movement of the contactor arm adjacent the terminal.

My invention may also be advantageously used to connect the individual sections of a socalled split Wound resistor. A resistor of this i type usually comprises several sections each of these sections. Also', because of the nickel content of the wire, the sections could not be satisfactorily soldered together.

In practicing thisembodiment of my vpres-v ent invention, the last few' turns of the one section and the first few turns of the adjacent section are embedded in a suitable soft metal, such as lead, which lead practically completely encases these turns, making a good electrlcal connection thereto and therebetween. Because of the completeness ofthe encasement of the turns in the metal, moisture is excluded to such an extent that corrosion is negligible and the good electrical contact between the two sections is thereby maintained over a long period of time. If desired, the resistance change per turn of the wires between these sections can be tapered by placing the soft metal piece used to connect the two sections together at an angle with respect to the longitudinal axis of the element so that diiferent portions of the turns are short circuited by being embedded in this metal.

Other objects of my invention, not specifif cally mentioned here will be apparent from a 'fied application reading ofthe detailed description and claims which follow.

Now, to acquaint those skilled in the art with the teachings of'my invention, reference 'is made to the accompanying drawing in which a preferred embodiment of it is shown by way of example, and in which:

Fi re 1 is a cross-sectional view longitudina ly through a resistance element embodying the teachings of, my invention showing the soft metal strip before it has been pressed over the wires of the element;

Figure 2 is a view similar to Figure 1 showing the soft metal pressed into engagement with the wires of the element;

Figure 3 is a similar Viewv showing a modiof the soft metal to the wires of the element; Y

VFigure 4 is an elevation view of the end of a reslstance element showing the applica- .tion of a tapered, soft metal piece thereto; l 60 Figure 5 is an elevational view ofa resistance elementV at the junction of two of its sections showing the application of a piece of soft metal thereto;

Figurel 6v is an elevational view of the end of an elementsimilar to that shown in Figure 4 showing another application of soft metal. i

the manner set out in my co-pending applica-v tion. The wires 2 and 3 are preferably space wound on the insulator 1, and prevented from moving longitudinallythereon infany preferr-ed manner.' As shown, the wires 2 are vance element and the wires 3, the first few turns of an adjacent element wound on the same insulator 1. In practice, the diameter of a winding 2 and 3 will be of the order of two or three thousandths of an inch, and the thickness of the insulator l approximately one thirty-second of an inch.

To connect the last few turns of the winding 2 to the first few turns of the winding 3, I have provided a piece of soft metal 4 which is laid over the windings 2.and 3,'and subsequently pressed to embed the turns of those windin into the metal 4 in the manner shown 1n Figure 2. Preferably, the element 4 comprises a piece of lead foil or small lead wire, although other suitable soft metal may be' emplo ed in lieu thereof. As will be noted in igure 2, after the piece of metal '4 has been pressed into the spaces between the wires 2 and 3, those wires are practically -completely encased in this metal. vAs a result, a good electrical contact is secured from the wire 2 to the metal 4 and from the metal 4 to the Wire 3, thereby oining the two sections 2 and 3 of the element electrically together. Furthermore since the wires are practically completely embedded in the metal, air and moisture are excluded from the electrical contact and corrosion is thereby reduced to such a point that it can be neglected, and the electrical connection relied upon for a long period of time. The metallic piece 4 projects beyond the outslde surface of the wires 2 and 3 for a very short `distance so that the spaceoccupied by the splice'between the two sections is very little greater than that occupied by the wires themselves.

In the embodiment of the invention shown in Figgres 1 and 2, the metallic piece 4 which may the last few turns of one section of a resistpractically completely embedded in the metal 5, a good electrical contact between the wires T and 3 is achieved and that contact will re.- main unchanged tor a long period of time.

lhe invention may also be applied to the end sections of a resistance element in the manner 'shown in Figures 4 and 6. lfn Figure 4, the end ot the winding 10 on the resistance on the insulator ll is embedded in a soft metal sheet 12, which sheet is shown as a. piece ot foil laid over the end turn ot the element l0 and pressed down Athcreagainst to embed those turns in it. As shown, the toil is provided with a tapered edge 13 which tapers the effective length ot the wire 10 fromV practically zero at the point 14 to the maximum possible length at the point 15. rllhis gradual increase in these effective lengths as the distance from the end ot the winding increases insures that the contactor of the rheostat into which the element is placed will move from the end of the element without encountering any large variations in resistance and consequently the rhcostat will have what is known as a smooth takeoff.

ln the embodiment shown in Figure 6 the end turns of the winding are held On the insulator' 21 by a sott met-al strip 22 which is a wire placed under the winding 2O and into which the winding is subsequently pressed. ln fabricating this embodiment of my invention, thesott metal wire 22 is placed adjacent to the strip 21 and the wire 20 wound thereover` the wire being wound under considerable tension which partially embeds it in the sott metal 22. The end of the strip is subsequently pressed to complete the embedding ot the wire 20 in the lmetal 22,

thereby serving to securely lock the end ,turnsY oit the wire 20 and to prevent them unwinding from the insulator 21. The wire 22 may be placed at an angle as shown 'to taper the effective resistance of the end turnotl the iyinding 2O in the manner hereinbeore pointe out.

Figure o shows a fragmentaryv elevationl oi" an element in which two windings and.

31 are joined by a soft metal member 32 in the manner shown in detail in Figures 1, 2 and 3, the metal 32 in this instance being as narrow as possible so that a minimum number of turns are short c ircuited by being embedded in it. In Figure 7, the windings 33 and 34 are joined by a soft metal Wire 35 preterably placed under them adjacent to the strip 36 before the wires 33 and 34 are Wound on it. The winding operation is made under considerable tension and partially embeds the l wires in the metal 35, and a subsequent pressing'securely embeds them in the metal. Because of the angled position oit' the wire 35 and consequent tapering of the effective lengths of the end turns of the windings 33 and 34, a smooth takeoff between the Windings 33 and 34 is assured. i

The wire and the wire 22 may also be placed down over the outside of their respective resistance wires, and the wires embedded therein by the application referred to in the same manner as the foils 12 and 32 are placed over their respective resistance wires. In certain instances within the teachings of my inventiomvthe ioils such as l2 and 32 may advantageously be placed under the Wire rather than over it as shown in the illustrative iigures.

The resistance elements inanutactured in this manner are simple and compact and therefore may be fabricated at minimum cost. The, particular method of holding the end turns oi the wire and the last turns of adjacent sections is at a further advantage in that it does not materially stitl'en the insulator upon which the wires are wound and consequently the insulator can be formed to an arcuate shape more readily. rlhe forming of the insulator te an arcuate shape does not disturb the bond between the soft metal and wire ot the element, and therefore does not impair the electrical connection between individual sections or loosen up lthc connec- .tion so that corrosion may take place within it.

In practicing the invention, l have found that a lead foil of approximately tive thousandths of an inch thickness works satisfactorily with nickel or alloy wires of from two to five thousandths ot an inch in dian'ieter` and elements manufactured in this manner have proven satisfactory in all instances. lWhile lead is preferable because of its low cost, other suitable sott metals may be advantageous in certain instances and may be substituted tor lead within the teaching of my invention.

Having thus described my invention, what I consider new and desire to have protected by Letters Patent is pointed out in the appended claims.

1. rl`l1e method of making a wire woundresistance element which comprises, winding the wire on an insulating strip, engagin the last few turns of' the wire with a piece o soft metal and pressing those turns into the metal.

2. The method of tapering the resistance value of the end section of a wire wound resistance unit which comprises pressing a piece of sott metal of tapering cross-section into the last few turns of the section to thereby reduce the eective length of the turns in steps as the end of the section isapproaehed.

3. A resistance element oomprising,an insulating strip, a resistance wire space wound thereon, and a soft metal foil engaging the end .turns of the wire, said metal foil being partly disposed-in the spaces between those turns to hold the wire on the strip. 4. A resistance element comprising, an 1nsulating strip, a resistance wire space wound 5 thereon, and a piece of soft lead engaging the end turns of the wire, said lead being partly disposed in the spaces between those v turns to hold the wire on the strip.

5. A resistance element comprising, an insulating strip, a resistance Wire spacewound thereon and a piece of lead of-varying crosssection engaging the end few turnsv of the wire, said lead being partly disposed in the spaces between those turns to hold the wire on the strip, and to taper the resistance value per turn as the end of the element is appreached.

'6. A resistance element comprising a ,plurality of turns of resistance wire wound on f an insulating strip, the end few turns of said wire being embedded into a strip of soft metal of tapering cross-section which holds those turns fromv unwinding from the strip and tapers the effective length of said turns. 7. A resistance element comprising an insulating strip, a plurality of turns of vresistance wire wound thereon, a second plurality of turns of resistancewire wound on the strip, the last few turns of the first plurality of turns being adjacent the iirst few turns of the second plurality of turns, a softmetallic member into which said adjacent turnsk are embedded to electrically connect them and to prevent them from unwinding from the strip.

8. A resistance element comprising an insulating strip, a resistance wire space wound thereon, anda soft metal foil engaging the end turns of the wire, said metal foil being angularly disposed in the spaces between those turns to short circuit the said end turns, whereby a low resistance progression is provided. n

In Witness whereof, I hereunto subscribe my-name this 25th day of April, 1930.

NEWTON C. SCHELLENGER.

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